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A highly contagious norovirus outbreak believed to have infected to at least 700 Oxnard-area elementary and middle school students is still wreaking havoc on stomachs and intestines, but its grip appears to be loosening.

All eight schools in the Rio School District, the epicenter of an outbreak that emerged about three weeks ago, still have students with norovirus symptoms. But the number of new cases has dropped significantly, said John Puglisi, superintendent of the district that encompasses more than 5,000 students in El Rio and north Oxnard.

"We're still on the decline," said Dr. Robert Levin, Ventura County public health officer.

Five schools in the Oxnard Elementary School District have reported 10 to 15 students who either have confirmed cases of the virus or are showing symptoms, said Chris Ridge, director of pupil services for the district of nearly 17,000 students. The cases have brought communication with parents, what Ridge called "deep cleaning" of classrooms that may have been contaminated and instructions for kids to sing their way through the alphabet while washing hands with soap and water.

"Handwashing is really the trick," he said. "The virus is typically passed by contact."

Concerns at Oak Park High School in east Ventura County emerged two weeks ago with a confirmed norovirus case followed by nearly a dozen students who showed symptoms. School officials took steps that included spending about $15,000 on cleaning efforts. Principal Kevin Buchanan said the norovirus tally has not grown.

"Right now, we have zero kids absent because of noroviruses," he said Wednesday.

Norovirus can be passed through human contact, contaminated surfaces and food. It is common enough to be blamed for 19 million to 21 million illnesses a year across the United States. It often occurs where people congregate in confined areas, spreading across cruise ships, nursing homes, day care centers and schools.

More on norovirus from the National Restaurant Association

Symptoms include cramping, nausea, vomiting and diarrhea. Symptoms can emerge from 12 hours to two days after exposure and last for one to three days.

People are most contagious when they are showing symptoms and during the first few days of recovery.

At Rio School District, preventive steps have included sending home students with symptoms, emphasizing hygiene and cleaning schools every night with recommended disinfectants. Puglisi said the special steps will likely continue until students leave for spring break on April 7.

Some school districts including Ventura Unified, Mesa Union in Somis and the Pleasant Valley School District in Camarillo reported that norovirus has not emerged.

Levin praised the Rio district's efforts and said the outbreak is in decline. He said there is no way to know whether the district is at the tail end of the sickness.